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Attention, Pupils!

The following questions appeared in the December 11, 1999, issue of the New Scientist:

Why do some animals have a non-circular pupil? Cats and some snakes have a vertical shape yet horses and goats have a horizontal one. What is the reason behind the differences and, more importantly, how do the different shapes affect how animals actually see things?

The answer for the vertical slit is that it improves an animal's focus in the direction perpendicular to the slit. Thus, cats and snakes hunting close to the ground can better detect their prey over a wide horizontal field.

But how about the horses, goats and other grazing animals that must keep their eyes open for predators across a wide horizon? Pupils with horizontal slits would seem to defeat this purpose. Ah, but when they lower their heads to graze, their vertical pupils become horizontal. Simple!

(Anonymous; "Eye to Eye," New Scientist, p. 85, December 11, 1999.)

Comments. But puzzles remain, otherwise we wouldn't address this subject. (1) Some snakes have round pupils (as in the illustration) having apparently been subjected to different environmental forces; (2) Of all the 9,000+ species of birds, only the skimmers own pupils with vertical slits. You would expect ground-feeding birds like robins and larks to also have them; (3) Most interesting are the cuttlefish with pupils shaped like Ws. That's a tough one.